1. Field of Invention
The present invention pertains to the field of printers. More particularly, this invention relates to a low cost ink drop detector.
2. Art Background
Prior printers including black and white printers and color printers commonly include one or more print heads that eject ink drops onto paper. Such a print head usually includes multiple nozzles through which ink drops are ejected. Typically, a print head ejects ink drops in response to drive signals generated by print control circuitry in the printer. A print head that ejects ink drops in response to drive signals may be referred to as a drop on demand print head.
One type of drop on demand print head employs piezo-electric crystals that squeeze out ink drops through nozzles in the print head in response to the drive signals. Another type of drop on demand print head employs heating elements that boil out ink drops through nozzles in the print head in response to the drive signals. Such print heads may be referred to as thermal ink jet print heads.
Typically, the nozzles through which ink drops are ejected can become clogged with paper fibers or other debris during normal use or clogged with dry ink during prolonged idle periods. Prior printers commonly include mechanisms for cleaning the print head and removing the debris. Such a mechanism may be referred to as print head service station and may include mechanisms for wiping the print head and applying suction to the print head to clear out any blocked nozzles.
Prior printers typically lack a mechanism for determining whether the print head actually requires cleaning. Such printers usually apply the service station to the print head based on a determination of whether the print head may possible require cleaning. Unfortunately, such printers must then employ over cleaning which usually slows the overall printing throughput.
It would be desirable to provide a printer with a mechanism for detecting whether ink drops are being ejected from the print head. Such a mechanism could be used to determine whether a print head actually requires cleaning. In addition, a mechanism for detecting ink drops could be used to detect permanent failures of individual nozzles which may be caused, for example, by failures of heating elements in a thermal ink jet print head.
One possible method for detecting the ejection of ink drops from a print head is to equip the printer with a drop detection station that employs piezo-electric material and associated circuitry which detects the impact of the ink drops hitting the detection station. Unfortunately, such piezo-electric material is relatively expensive and adds to the manufacturing cost of a printer. In addition, such a mechanism usually cannot detect extremely small ink drops as are used in high resolution and color printers. Moreover, piezo-electric material typically loses sensitivity as ink accumulates on its surface thereby reducing its ability to detect ink drop impacts.
Another possible solution is to equip the printer with an optical detector that includes a light source and a detector. Typically, an ink jet nozzle must be aimed so that ink drops pass between the light source and the detector and occlude light rays that travel between the light source and the detector. Unfortunately, the circuitry for such an optical detector is usually expensive and therefore adds to the manufacturing cost of a printer. In addition, such a technique usually requires very fine control over the positioning of the optical detector with respect to nozzles being tested. Moreover, mist or spray from the nozzle can contaminate the optical detector and cause reliability problems.
Another possible solution which is specific to thermal ink jet print heads is to equip the print head itself with an acoustic detector. Typically, such an acoustic drop detector detects the shock wave associated with the collapse of ink bubbles in the print head. Unfortunately, such ink bubble shock waves may occur even though ink is not being ejected from the print head. In addition, acoustic measurements can be corrupted by large current pulses that occur during printer operation. Moreover, the acoustic detector and associated signal amplifier circuitry for such an acoustic detector is usually expensive and increases the overall manufacturing costs of a printer.